1. Field of the Invention
This invention relates to antisera produced by novel antigens, which are useful in testing for pregnancy in humans and more particularly to antisera having an immunological reaction with human chorionic gonadotropin, wherein the immunological cross-reactivity with human luteinizing hormone is reduced or eliminated. This further relates to the production of antigens to prepare said antisera, and which may also be used for contraceptive purposes to terminate human pregnancy.
2. Description of the Prior Art
Human chorionic gonadotropin is a hormone produced by the placenta during pregnancy. Presence of the hormone in the serum and urine therefore serves as an indication of pregnancy. The presence of this hormone has been detected by its effect on the ovaries of animals and, more recently, by immunoassay.
However, because the tests used hitherto do not sufficiently distinguish between hCG and other hormones which are present, such as luteinizing hormone, the presence of hCG cannot be unambiguously detected until several weeks after conception, by which time the levels of hCG are high enough so that it can be detected even in the presence of interfering substances.
The use of sensitive procedures such as radioimmunoassay has not been practical because the antisera to hCG hitherto available have been reactive not only with hCG, but also with other hormones which are present, such as human luteinizing hormone (hLH), follicle stimulating hormone, and thyroid stimulating hormone. This occurs because isoimmunization with hCG results in the production of antibodies which cross-react with hLH, follicle stimulating hormone, and thyroid stimulating hormone. These hormones resemble one another in consisting of two non-covalently bonded subunits, .alpha. and .beta.. Whereas the .alpha.-subunits are almost identical in all of them, the .beta.-subunits are hormone specific and are structurally dissimilar. Use of the .beta.-subunit of hCG (hCG-.beta.) as an antigen produces an antiserum having improved specificity, but which still shows appreciable cross-reactivity with hLH. The molecular basis of this cross-reactivity is the presence of considerable homology in the amino acid sequences of hCG-.beta. and hLH-.beta., especially in the amino terminal 75% of the molecule. The carboxy-terminal 32-residue peptide is unique to hCG-.beta.. The amino terminal 75% of the molecule contains 6 intrachain disulfide linkages which maintain the conformation of the molecule. This conformation is believed to be responsible for much of the antigenic activity of hCG as well as its immunological cross-reactivity with hLH.
In addition to its use as an indication of pregnancy, hCG is necessary for the maintenance of pregnancy. Hence, if the hormone can be neutralized, the pregnancy can be terminated. The neutralization of hCG can be the basis of a contraceptive vaccine. However, raising antibody to hCG in humans presents two major problems. hCG is a human hormone, and humans normally will not produce antibody to a human hormone. This limitation can be overcome by modifying hCG in such a way that it is not recognized by the human system as its own hormone and behaves as a foreign material. There is still another problem. The antibody to hCG would also neutralize another hormone, namely luteinizing hormone (LH) from the anterior pituitary gland, which is necessary for the normal human reproductive cycle. This problem can be partially overcome by using one of the two components of hCG molecule, designated as the .beta.-subunit, the antibody to which would predominantly neutralize hCG. However, it still lacks the desired specificity.
Pappenhagen et al, U.S. Pat. No. 3,903,262, disclose modifying serum globulins by reducing and cleaving disulfide bonds and subsequently alkylating the cleaved disulfide bonds for the purpose of reducing the anticomplement activity of such globulins. However, Pappenhagen does not disclose modifying serum globulins for the purpose of obtaining antibody selectivity for one hormone over another, nor does he disclose conjugation of globulins with other proteins.
The procedure of conjugating the .beta.-subunit of human chorionic gonadotropin with tetanus toxoid to reduce cross-reactivity with other hormones, including follicle-stimulating hormone, thyroid-stimulating hormone, and luteinizing hormone, is allegedly disclosed by Talwar, G. P., et al, Proc. Natl. Acad. Sci. U.S.A. 1976 73(1), 218-222. However, no disclosure of reducing and cleaving disulfide bonds is found in this article.
Bahl, O. P. et al, Biochem. Biophys. Res. Comm. 70, 525-532 (1976), disclose modifying the .beta.-subunit of human chorionic gonadotropin by reducing and cleaving disulfide bonds and alkylating the reduced and cleaved disulfide bonds. These derivatives of hCG-.beta. have reduced immunological reactivity with hCG, but show a greater loss in their immunological cross-reactivity with human luteinizing hormone. Those derivatives in which 3-5 of the total of 6 intrachain disulfide bonds are reductively cleaved and alkylated are reported to produce antibodies reactive with hCG. The derivative having all 6 disulfide bonds reductively cleaved and alkylated is reported to be immunologically inactive.
This article also discloses that the specificity of hCG having 3 disulfide bonds and S-alkylated can be further enhanced by conjugation with tetanus toxoid. In contradiction to the above report of Talwar et al, this article reports that conjugation of the unmodified hCG-.beta. with tetanus toxoid does not preferentially reduce its immunological cross-reactivity with human luteinizing hormone.
Therefore, a need exists for a derivative of human chorionic gonadotropin which can act as an antigen to stimulate the formation of antibodies specifically to hCG with reduced or eliminated immunological cross-reactivity to hLH. Such a derivative would be useful both in preparing antisera for use in pregnancy testing by immunoassay and as a vaccine for preventing or terminating pregnancy.